Fence sensor

ABSTRACT

It is an object of this invention to provide a fence sensor which can achieve an excellent detection stability without using a current for generating an electric field nor infrared rays. The fence sensor comprises a detection electrode ( 8 ); a reference electrode ( 9 ) insulated from the detection electrode ( 8 ); a chargeable member ( 4 ) insulated from both of the detection electrode ( 8 ) and the reference electrode ( 9 ); and a detection circuit ( 20 ) for detecting a change in the electrostatic capacitance between the detection electrode and the reference electrode that is generated by the presence of an object to be detected within a detection region of the detection electrode. In this fence sensor, the chargeable member is arranged such that at least a part of the chargeable member is situated within the detection region of the detection electrode. Further, the chargeable member is formed from a conductor.

FIELD OF THE INVENTION

The present invention relates to a fence sensor, and more particularlyto a crime prevention sensor for a fence which can detect the presenceof an object that approaches or makes contact with the fence.

BACKGROUND OF THE INVENTION

Among the conventional crime prevention sensors for a fence fordetecting an intruder, there has been known an electric field formationtype sensor as disclosed in Japanese Patent Laid-Open Publication No.9-237389. This is a type in which an electric field is generated bysupplying sinusoidal current to electric wires embedded in the fence,and an alarm device is actuated when a change of the electrostaticcapacity is detected that occurs due to an intrude approaching thefence.

In addition, there has also been known a crime prevention sensor inwhich an emitting part of infrared rays and a receiving part forreceiving the emitted infrared rays are installed in the vicinity of thefence in order to establish an infrared ray detection region along thefence. In this sensor, when an interception of infrared rays by anintruder is detected, an alarm device is actuated.

The electric field formation type sensor uses a sinusoidal wave togenerate an electric field, so it has a problem that it becomes a sourceof noise in telephone lines or electronic circuits found in the vicinityof the fence. Accordingly, installation sites for the sensor arelimited.

Moreover, the electric field formation type sensor has a detectionregion defined by the region where electric wires are laid within thefence, so that it has a problem that it gives restrictions on the fencedesign.

Besides, the electric field formation type sensor has another problemthat the power consumption increases due to the necessity that theelectric field has to be maintained all the time.

Furthermore, with respect to the infrared sensor, the detection regionbetween the light emitting part and the light receiving part needs to beformed in a linear shape, so that it has a problem that a detectionregion cannot be formed along a fence having a curved surface structure.

It is an object of the present invention to provide a fence sensor withan excellent detection stability that resolves the problems associatedwith the electric field formation type sensor and the infrared sensordescribed above without requiring electric current for generating anelectric field or infrared rays.

SUMMARY OF THE INVENTION

A fence sensor defined by claim 1 comprises:

a detection electrode;

a reference electrode insulated from the detection electrode;

a chargeable member insulated from both of the detection electrode andthe reference electrode, the chargeable member being arranged such thatat least a part of the chargeable member is situated within a detectionregion of the detection electrode, and the chargeable member beingformed from a conductor; and

a detection circuit for detecting a change in the electrostaticcapacitance between the detection electrode and the reference electrodethat is generated by the presence of an object to be detected within thedetection region.

In this structure, when the electrical charges on the chargeable memberare increased by the presence of the object within the detection regionof the chargeable member, the electrostatic capacitance between thedetection electrode and the reference electrode is changed. According tothe present invention described above, the fence sensor can detect thepresence of the object within the detection region by detecting such achange in the electrostatic capacitance between the detection electrodeand the reference electrode. Accordingly, the fence sensor of thisinvention does not require the formation of an electric field or the useof infrared rays.

For example, if an intruder approaches the detection region,electrostatic induction is generated in the conductive chargeable memberdue to the charge on the body of the intruder, thus increasing theamount of the charge on the chargeable member. Since the chargeablemember is insulated from both of the detection electrode and thereference electrode, the charges on the chargeable member will not movedirectly to these electrodes as currents. However, since the chargeablemember is found within the detection region of the detection electrode,the increase in the charges on the chargeable member forms an electricfield in the detection region of the detection electrode, and causes anincrease in the charge on the detection electrode. Consequently, theelectrostatic capacitance between the detection electrode and thereference electrode is increased. When the increase in the electrostaticcapacitance exceeds a detection threshold of the detection circuit, thedetection circuit outputs a detection signal.

Further, the use of the chargeable member makes it possible to form adetection region with wider area. For example, when a wide area of thesidewall of the fence is formed as a detection region, the increase inthe detection region can simply be achieved by the installation of achargeable member on the entire sidewall of the fence. However, if thedetection electrode and the reference electrode are installed over awide area, the electrostatic capacitance between the detection electrodeand the reference electrode becomes extremely large in the case ofabsence (static state) of an object in the detection region of thedetection electrode.

When the electrostatic capacitance between the detection electrode andthe reference electrode is extremely large as in the above, theincreased amount of the charge on the detection electrode in the chargedstate (that is, in a state that an object is found within the detectionregion) will be relatively extremely small compared with the amount ofthe charge in the static state. Because of this, the detection circuithas to detect an extremely large increase, relatively speaking, in theamount of the charge, thus impairing detection stability or causinginability of detection. For these reasons, the fence sensor of thisinvention utilizes the chargeable member. According to such a fencesensor, it is possible to stably detect an object within a widedetection region. Further, it is also possible to achieve the detectionwithout being accompanied by an increase in the electrostaticcapacitance between the detection electrode and the reference electrodein the static state.

Here, there is no limitation on a fence in which the fence sensor ofthis invention is to be installed. Examples of such a fence includes awall formed of concrete or stone; a palisade formed from support pillarsarranged with a prescribed distance apart and a metallic net spreadbetween the support pillars; and the like. Further, such a fence may beinstalled indoors, and may also be installed outdoors. In addition, thefence sensor of this invention may be installed in a fence so that thedetection region covers the entirety of the fence. Further, the fencesensor may also be installed so that the detection region covers a partof the fence (e.g., handrails of the fence).

Further, the use of the fence sensor is not limited to the purpose ofcrime prevention. For example, a fence sensor of this invention may beinstalled on a fence in the rear of a parking lot in order to give awarning about the approach of a vehicle to the fence. According to sucha fence sensor, it is possible to prevent collision of a vehicle withthe fence.

The fence sensor defined by claim 2 further comprises water filmseparation means for separating a water film on the surface of thechargeable member from a water film grounded to the earth.

A fence sensor defined by claim 3 comprises:

a detection electrode;

a reference electrode insulated from the detection electrode;

a chargeable member arranged such that at least a part of the chargeablemember is situated within a detection region of the detection electrode,the chargeable member being formed from an insulator; and

a detection circuit for detecting a change in the electrostaticcapacitance between the detection electrode and the reference electrodethat is generated by the presence of an object to be detected within thedetection region.

When an intruder approaches the chargeable member (which is aninsulator), induced polarization is generated in the chargeable memberdue to the charges on the body of the intruder. Then, an electric fieldis generated in the detection region of the detection electrode bypolarized charges generated by the induced polarization. As a result ofthe formation of the electric field, the electrostatic capacitancebetween the detection electrode and the reference electrode isincreased, and the detection circuit detects the presence of theintruder.

Here, there is no limitation on material used for making the insulator,and shape of the insulator. For example, the insulator may be preparedusing material such as wood, a synthetic resin, stone, earthenware,concrete, and the like.

In the fence sensor defined by claim 4, the detection electrode and thereference electrode are partially or completely concealed by thechargeable member.

Since the fence sensor of this invention has the detection electrode andthe reference electrode which are concealed by the chargeable member, ittends to be difficult to make approach to or direct contact with boththe electrodes. Because of this, according to the present invention, itis possible to prevent breakdown of the detection circuit byelectrostatic sparks. This arrangement is made for avoiding thefollowing undesirable case. That is, when the air is dry, the chargequantity on the body of the intruder is extremely large. In such acondition, if both the detection electrode and the reference electrodeare exposed, electrostatic sparks are generated between the electrodesand the human body, and its high voltage current will instantly destroythe detection circuit that is connected to these electrodes.

Further, according to the fence sensor of this invention, the chargeablemember is insulated from both the detection electrode and the referenceelectrode. Consequently, it is possible to make the chargeable memberabsorb the high voltage current by the electrostatic sparks, thus makingit possible to prevent a high voltage current from directly flowing tothe detection circuit. In addition, the fence sensor of this inventionhas an advantage that an easy revealing of the presence of the sensorcan be avoided because of the concealment of the electrodes by thechargeable member.

In this invention, the detection electrode and the reference electrodeare partially concealed. This means to conceal only a portion that has ahigh possibility of making approach to or contact with the human body,that is, to conceal only a portion where electrostatic sparks tend to begenerated between the electrodes and the human body. In addition, itmeans to conceal only portions that make these electrodes readilyidentifiable from the outside.

In the fence sensor defined by claim 5, the reference electrode isconnected electrically to the ground or a building.

When the reference electrode is connected to the ground, it is possibleto set the detection threshold higher, since the electrostaticcapacitance between the detection electrode and the reference electrodein the charged state can be increased compared with the case where it isnot connected to the ground. Therefore, the ratio of the signal to noisegenerated by the environment (i.e., S/N ratio) can be enhanced, and thedetection stability can be improved.

Here, the connection with the ground means to connect the referenceelectrode to the ground in the case where the fence is installed on theground. Further, the connection with the building also means to connectthe reference electrode to the building in the case where the fence isinstalled in the terrace or the like of the building. In thisconnection, the electrical connection will not involve the use of agrounding resistance as a necessary condition.

In the fence sensor defined by claim 6, the chargeable member isprovided with water repellent means.

For example, in the case where the chargeable member is made of concretewhich is an insulator, if moisture infiltrates into the chargeablemember, the migration of charges within the chargeable member isfacilitated by the hydrogen ions that are charged positively, and thusthe chargeable member is converted to a state that resembles to that ofa conductor.

Accordingly, the rate of increase of the electrostatic capacitance ofthe detection electrode in the static state and the electrostaticcapacitance in the charged state decrease relatively. Because of this,the detection circuit has to detect the increase rate that is relativelydecreased, and it becomes necessary to enhance the detection precision.

For this reason, in this invention the water repellent means is providedin the chargeable member in order to prevent infiltration of moistureinto the interior of the chargeable member. This makes it possible tokeep the charge quantity on the chargeable member in the static state,and thus a high detection precision is maintained.

The fence sensor defined by claim 7 further comprises directivitycontrol means for limiting the direction of the electric lines of forceof the detection electrode.

According to the fence sensor having such directivity control means, forexample, it is possible for the fence sensor to detect an intruder whotries to jump over the fence, and also possible to avoid detecting apedestrian who passes along the fence.

In the fence sensor defined by claim 8, the directivity control means isa shielded electrode connected to the reference electrode. This fencesensor has the shielded electrode connected to the reference electrode,so that it is possible to completely shield off unwanted electric linesof force of the detection electrode.

The fence sensor defined by claim 9 further comprises at least oneinter-electrode chargeable member which is disposed between thedetection electrode and the reference electrode, and which is insulatedfrom both of the detection electrode and the reference electrode. Thisfence sensor is capable of stabilizing the sensitivity of the detectionelectrode, and reduce the detection threshold of the detection circuitby equipping it with the inter-electrode chargeable members.Accordingly, the detectable region of the sensor can be extended.

In the fence sensor defined by claim 10, the detection electrodeincludes first and second detection electrodes which are insulated fromeach other, and the detection circuit includes comparison means forcomparing electrostatic capacitance between the first detectionelectrode and the reference electrode with electrostatic capacitancebetween the second detection electrode and the reference electrode.

In the fence sensor defined by claim 11, the detection electrode and thereference electrode are constructed from a plurality of sets ofdetection electrode and reference electrode, in which the plurality ofthe detection electrodes are electrically connected to each other, theplurality of the reference electrodes are electrically connected to eachother, and the plurality of detection electrodes and the plurality ofthe reference electrodes are connected to the detection circuit. Thisfence sensor can realize a wide range of detectable region at a low costby detecting the changes in the electrostatic capacitances between theplurality of detection electrode and reference electrode sets using asingle detection circuit.

A fence sensor defined by claim 12 comprises:

a detection electrode;

a reference electrode insulated from the detection electrode;

a detection circuit for detecting a change in electrostatic capacitancebetween the detection electrode and the reference electrode that isgenerated by the presence of an object to be detected within thedetection region; and

a capacitor connected in series between the detection circuit and thedetection electrode, the capacitor being disposed separated from thedetection electrode.

The fence sensor defined by claim 13 further comprises electrostaticspark preventing means which is disposed between the detection circuitand the detection electrode.

A fence sensor defined by claim 14 comprises:

a detection electrode;

a reference electrode insulated from the detection electrode;

a detection circuit for detecting a change in electrostatic capacitancebetween the detection electrode and the reference electrode that isgenerated by the presence of an object to be detected within thedetection region; and

water film separation means for separating a water film on the surfaceof the detection electrode from a water film grounded to the earth.

In the fence sensor defined by claim 15, the water film separation meanshas a trench with a width of 6 mm or more, in which the trench is openeddownward.

In the fence sensor defined by claim 16, the water film separation meanshas a main trench with a width of 6 mm or more and an auxiliary trenchwith a width of less than 6 mm, in which the main trench is openeddownward, and the auxiliary trench is opened downward and provided inthe main trench.

A fence sensor defined by claim 17 comprises:

a detection electrode;

a reference electrode insulated from the detection electrode; and

a chargeable member insulated from both of the detection electrode andthe reference electrode, the chargeable member being formed from aconductor or an insulator, and the chargeable member being arranged suchthat at least a part of the chargeable member is situated within adetection region of the detection electrode.

A sensor for an ascending and descending member defined by claim 18comprises:

a detection electrode;

a reference electrode insulated from the detection electrode;

an ascending and descending member insulated from both of the detectionelectrode and the reference electrode, the ascending and descendingmember being formed from a conductor or an insulator, and the ascendingand descending member being arranged such that at least a part of theascending and descending member is situated within a detection region ofthe detection electrode; and

a detection circuit for detecting a change in electrostatic capacitancebetween the detection electrode and the reference electrode that isgenerated by the presence of an object to be detected within thedetection region.

The sensor for an ascending and descending member according to thepresent invention belongs the same technical field and has the sameproblems to be solved as those of the fence sensor described above.Therefore, this fence sensor is also capable of detecting an intruder orthe like who steps on the ascending and descending member that is achargeable member. In this sensor, a change in the electrostaticcapacitance between the detection electrode and the reference electrode,generated by the presence of an object to be detected within a detectionregion, is detected by the detection circuit.

Examples of the ascending and descending member include a ladder and anemergency stairway, and this ascending and descending member mainlydesignates a member on which a human being steps when ascending anddescending. For example, in the case of a ladder, a rung of the ladderis made of a stainless steel pipe, and two electric wires forming adetection electrode and a reference electrode are strung inside therung. In the case of an emergency stairway, a step is made of concrete,and an electrode member having two conductive layers forming a detectionelectrode and a reference electrode is attached to the rear surface ofthe step.

A sensor for an ascending and descending member defined by claim 19comprises:

a detection electrode;

a reference electrode insulated from the detection electrode; and

a chargeable member insulated from both of the detection electrode andthe reference electrode, the chargeable member being formed from aconductor or an insulator, and the chargeable member being arranged suchthat at least a part of the chargeable member is situated within adetection region of the detection electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial perspective view which shows a first embodiment ofthe fence sensor according to the present invention, in which a holdmember 10 is left out to show clearly a detection electrode 8 and areference electrode 9;

FIG. 2 is a sectional view along line A—A in FIG. 1;

FIG. 3 is a circuit diagram which shows a detection circuit 20 of thefence sensor in FIG. 1;

FIG. 4 is a partial perspective view which shows a second embodiment ofthe fence sensor according to the present invention;

FIG. 5 is an enlarged view which shows the cross-section of the end partof the fence in FIG. 4;

FIG. 6 is a partial perspective view which shows a third embodiment ofthe fence sensor according to the present invention;

FIG. 7 is a sectional view along line B—B in FIG. 6;

FIG. 8 is a circuit diagram which shows a detection circuit 90 of thefence sensor in FIG. 6;

FIG. 9 is a block diagram which shows a fourth embodiment of the fencesensor according to the present invention;

FIG. 10 is a partial perspective view which shows a fifth embodiment ofthe fence sensor according to the present invention;

FIG. 11 is a partial perspective view which shows a sixth embodiment ofthe fence sensor according to the present invention;

FIG. 12 is an explanatory diagram which shows a seventh embodiment ofthe fence sensor according to the present invention, in which thepositional relationship between a support pillar and a guardrail of thefence sensor is shown;

FIG. 13 is a sectional view along line A—A in FIG. 12;

FIG. 14 is a sectional view along line B—B in FIG. 12;

FIG. 15 is a sectional view along line C—C in FIG. 12;

FIG. 16 is an explanatory diagram which shows an eighth embodiment ofthe fence sensor according to the invention, in which the positionalrelationship between a support pillar and a chains of the fence sensoris shown;

FIG. 17 is a perspective view which shows a ninth embodiment of thefence sensor according to the present invention; and

FIG. 18 is a longitudinal view which shows a detection leg part 170 inFIG. 17.

PREFERRED EMBODIMENTS OF THE INVENTION

A fence sensor according to a first embodiment of the present inventionwill be described with reference to FIGS. 1-3. The fence sensor of thisembodiment is used as a sensor for a crime prevention system installedin a fence built on a terrace of an apartment house. This fence sensoris designed to give an alarm when an intruder approaches the handrail.

Reference numeral 1 represents a fence, which has a fence main body 2and a handrail 3 provided on the upper surface of the fence main body 2.The handrail 3 includes a stainless steel hollow pipe 4, a stainlesssteel support pillar 5, and an insulating member 11 made of a syntheticresin for insulating the pipe 4 from the support pillar 5. An electrodemember 7 is extended in the length direction of the pipe 4 in theinternal space 6 of the pipe 4 that serves as a chargeable member.

The electrode member 7 consists of a detection electrode wire 8, areference electrode wire 9 extending in parallel to the detectionelectrode wire 8, and a hold member 10 which holds the distance betweenthese electrode wires. Inside the space 6, a fixing member (not shown inthe drawings) for fixing the electrode member 7 at a predeterminedposition is provided.

Next, the overall structure of the fence sensor of this embodiment willbe described. The detection electrode wire 8 is connected to a detectioncircuit 20 shown in FIG. 3, and the reference electrode wire 9 isgrounded to the body of the apartment house. In addition, electrodemembers 13 and 16 provided in fences of other terraces of the sameliving section of the apartment house are also connected to thedetection circuit 20. Namely, three detection electrode wires 8, 14, and17 are connected in parallel, and three reference electrode wires 9, 15,and 18 are connected in parallel. These electrode wires share onedetection circuit 20.

The detection circuit 20 is connected to a control circuit (not shown inthe drawings). Upon receipt of a detection signal from the detectioncircuit 20, the control circuit causes an indoor loudspeaker (not shownin the drawings) connected to the control circuit to generate an alarmsound as well as causes a lighting system (not shown in the drawings) inthe terrace to flicker.

Next, the detection circuit 20 will be described in more detail withreference to FIG. 3. The detection circuit 20 includes a pulse signalgenerating circuit 21, a differential amplifier 22, an AC/DC converter23• and a comparator 24 that are connected in series. A pulse signal V1output from the pulse signal generating circuit 21 is branched, and thewave form of the branched pulse signal is dulled due to a resistor 25and an increase in the electrostatic capacitance of the electrode member7.

The differential amplifier 22 amplifies the voltage difference betweenthe pulse signal V1 and a pulse signal V2 formed by the change in theelectrostatic capacitance, and the output V3 from the differentialamplifier 22 is converted to a DC voltage by the converter 23. Thecomparator 24 compares the output V4 from the converter 23 with apredetermined detection threshold, and sends out a detection signal tothe control circuit in the case where V4 is larger than the threshold.

Here, in order to position the inner wall surface of the pipe 4 withinthe detection region of the detection electrode wire 8, either thethreshold or the distance from the detection electrode wire 8 to theinner wall surface of the pipe 4 is adjusted.

Next, operations of the fence sensor of this embodiment will bedescribed. As an intruder approaches the pipe 4, electrostatic inductionis induced in the pipe 4 by the charges on the body of the intruder. Thecharges on the pipe 4 increased by the electrostatic induction form anelectric field on the inner wall surface of the pipe 4.

Since the pipe 4 and the support pillars 5 are insulated from each otherby the insulating member 11, the increased charges on the pipe 4 willnot move to the body of the apartment house via the support pillars 5.In the meantime, if the air is dry and the charge quantities on the bodyof the intruder are extremely large, electrostatic sparks will begenerated between the pipe 4 and the body of the intruder.

However, the high voltage current of the sparks will be discharged fromthe surface of the pipe 4 to the upper edge 12 of the support pillar 5,and flow to the body of the apartment house. Accordingly, the highvoltage current will not flow directly to the detection electrode wire 8and the reference electrode wire 9 which are totally concealed by thepipe 4 that acts as a chargeable member. Because of this, the detectioncircuit 20 will not be destroyed by the generation of electrostaticsparks.

An electrostatic induction will be generated in the detection electrodewire 8 by the above-mentioned formation of the electric field, and theelectrostatic capacitance between the detection electrode wire 8 and thereference electrode wire 9 will be increased by the electrostaticinduction. The increase in the electrostatic capacitance is detected bythe detection circuit 20. By the transmission of a detection signal fromthe detection circuit 20, the control circuit generates a warning soundfrom a loudspeaker and flickers the lighting system in the terrace toinform the residents of the approach of an intruder.

Next, a second embodiment of the fence sensor according to the presentinvention will be described with reference to FIGS. 4 and 5. In thefollowing description, it will be assumed that the detection circuit ofthis embodiment utilizes the detection circuit 20 as described in thefirst embodiment. Further, it will also be assumed that the controlcircuit sounds an alarm announcement from a loudspeaker and flickers alighting system in the garden upon receipt of a detection signal fromthe detection circuit 20.

In a plurality of electrode members 40 and one electrode member 100(that will be described later), detection electrode layers 41 and 101 ofare connected in parallel, and reference electrode layers 43 and 103 areconnected in parallel. These electrode layers share one detectioncircuit 20. Further, the reference electrode layers 43 and 103 aregrounded through grounding resistors.

Reference numeral 30 shows a fence provided along the border line oflot. The fence 30 includes a plurality of electrode members 40, oneelectrode member 100, aluminum support pillars 31 erected with aprescribed spacing, an aluminum pipe 32 supported at the upper end ofthe support pillars 31 via a synthetic resin insulating member 33,frames 34 supported on the side faces of the support pillars 31 via analuminum support member 36, and aluminum palisades 35 fixed to theframes 34.

Insulating members 37 are disposed at both ends of the support member 36to insulate the support pillars 31 from the frames 34. The pipe 32 isinstalled separated from the frames 34, and its lower surface is cut outin the length direction to form an open part 47. In other words, thesectional form of the pipe 32 has a horseshoe shape.

Each electrode member 40 is formed into a roughly rectangularparallelepiped shape. Further, each electrode member 40 is formed bylaminating the detection electrode layer 41, an inter-electrode chargelayer 42, the reference electrode layer 43, and insulating members 44and 45. In this structure, the insulating members 44 and 45 arerespectively interposed between the detection electrode layer 41, aninter-electrode charge layer 42, and the reference electrode layer 43.Furthermore, each electrode member 40 is fixed to the lower surface of aprism-shaped insulating member 46 that is fixed to the inner wall sideface of the pipe 32 at its one end and that insulates the referenceelectrode layer 43 from the pipe 32. Besides, each electrode member 40is disposed such that the detection electrode layer 41 faces an uppersurface 48 of the corresponding frame 34 a predetermined distance apart.

The electrode member 100 is formed into a roughly rectangularparallelepiped shape, and is formed by laminating the detectionelectrode layer 101, an inter-electrode charge layer 102, and thereference electrode layer 103 by interposing insulating members 104 and105. Further, the electrode member 100 is fixed to the upper surface ofthe prism-shaped insulating member 46 that is fixed to the inner wallsurface of the pipe 32 at its one end, and that insulates the referenceelectrode layer 103 and the pipe 32. Besides, the electrode member 100has the detection electrode layer 101 arranged with a prescribeddistance apart from an inner wall upper surface 106 of the pipe 32.

The inter-electrode charge layer 42 is insulated from each of thedetection electrode layer 41 and the reference electrode layer 43, andis not electrically connected to any other member. The charge layer 42supplies charge to or absorbs charge from the detection electrode layer41 depending upon the charge quantity on the detection electrode layer41. Namely, the charge layer 42 functions as a supply and absorb part ofcharge for the detection electrode layer 41.

Specifically, since two capacitors connected in series are formed by thedetection electrode layer 41, the charge layer 42 and the referenceelectrode layer 43, the electrostatic capacitance of the area isdecreased. Because of this, the changes in the electrostatic capacitancebetween the detection electrode layer 41 and the reference electrodelayer 43 caused by the changes in the environment (e.g., changes in thetemperature, humidity, radio waves, vibrations or the like), namely,noises can be reduced.

The provision of the charge layer 42 enables the ratio of the signal tonoise generated by the environment (i.e., S/N ratio) to be increased,thus making it possible to maintain a stabilized detection sensitivityof the electrode member 40. As a result, the detection threshold of thedetection circuit 20 can be set low, and a detection region R1 of thedetection electrode layer can be extended.

Similarly, the inter-electrode charge layer 102 is insulated from eachof the detection electrode layer 101 and the reference electrode layer103, and is not electrically connected to any other members, so that itfunctions as a supply and absorb part of charges for the detectionelectrode layer 101.

In this embodiment, each of the frame 34, the palisade 35 and the pipe32 constitutes a chargeable member. In other words, as shown in FIG. 5,the upper surface 48 of the corresponding frame 34 is situated withinthe detection region R1 of the detection electrode layer 41 of eachelectrode member 40. The detection region R1 is formed inside the openpart 47.

Further, the upper surface of the inner wall of the pipe 32 is situatedwithin a detection region R2 of the detection electrode layer 101 of theelectrode member 100. In this connection, it is to be noted that thedetection regions R1 and R2 shown in FIG. 5 respectively indicate thedetection regions of the detection electrode layers 41 and 101 whenchargeable members such as the frame 34 and the pipe 32 do not exist.Further, it is also to be noted that the range over which the body of anintruder can induce a prescribed amount of charge on the chargeablemember such as the frame 34 and the pipe 32 is represented by adetection region R3.

Next, operations of the fence sensor of this embodiment will bedescribed. When an intruder approaches the pipe 32 to jump over thefence 30, electrostatic induction is induced by the charges on the bodyof the intruder. The increased charges on the pipe 32 due to theelectrostatic induction form an electric field in the upper surface ofthe inner wall of the pipe 32.

Since the pipe 32 is insulated from the support pillar 31, the chargesincreased by the electrification do not migrate to the ground via thesupport pillar 31. In the meantime, since the detection electrode layer101 and the reference electrode layer 103 are concealed by the pipe 32which acts as a chargeable member, a high voltage current generated bythe electrostatic sparks will not flow directly to the detection circuit20.

Since the above-mentioned electric field is formed within the detectionregion R2 of the detection electrode layer 101, an electrostaticinduction will be induced in the detection electrode layer 101 by theapproach of an intruder. The electrostatic capacitance between thedetection electrode layer 101 and the reference electrode layer 103 isincreased by the electrostatic induction, and the increase in theelectrostatic capacitance is detected by the detection circuit 20. Whenthe detection circuit 20 sends out a detection signal, the controlcircuit causes the loudspeaker to issue a warning announcement andcauses the lighting system in the garden to flicker to inform theresidents of the approach of the intruder.

Next, if an intruder approaches the pipe 30 by crawling from sideway ofthe fence 30, electrostatic induction is generated in the palisade 35.The increased charges on the palisade 35 due to the electrostaticinduction migrate to the frame 34, and form an electric field on theupper surface 48 of the frame 34.

Since the frame 34 is insulated from the support pillar 31, the chargesincreased by the electrification do not migrate to the ground via thesupport pillar 31. In the meantime, since the detection electrode layer41 and the reference electrode layer 43 are concealed by the pipe 32, ahigh voltage current generated by the electrostatic sparks will not flowdirectly to the detection circuit 20.

Since the electric field is formed within the detection region R1 of thedetection electrode layer 41, electrostatic induction is generated inthe detection electrode layer 41. The electrostatic capacitance betweenthe detection electrode layer 41 and the reference electrode layer 43 isincreased by the electrostatic induction, and the increase is detectedby the detection circuit 20. When the detection circuit 20 sends out adetection signal, the control circuit causes the loudspeaker to issue analarm warning by voice and causes the lighting system in the garden toflicker to inform the residents of the approach of an intruder.

According to this embodiment, it is possible to make the entirety of thefence as a detection region by simply installing electrode members atappropriate places of the fence. Because of this, it eliminates the needfor laying around electric wires on the inside of the fence, as wasrequired in the conventional sensor of electric field formation type.Further, it is possible to minimize the restrictions when designing thefence.

Next, a third embodiment of the fence sensor according to the presentinvention will be described with reference to FIGS. 6-8. Referencenumeral 50 indicates a concrete fence installed along the border line oflot. This concrete fence 50 includes an electrode member 60, a wall body51 having sidewalls 52 and 53, and a concrete chargeable member 54 fixedto the upper part of the wall body 51 via an insulating member 55 whichis a filling material made of a synthetic resin.

The electrode member 60 is housed in a trench provided in the upper partof the wall body 51 so as to be disposed in the length direction of thewall body 51 and the chargeable member 54. This electrode member 60 isarranged between the wall body 51 and the chargeable member 54. Further,the electrode member 60 includes a case 61 made of a synthetic resin,and a first electrode member 70 and a second electrode member 80 housedin the case 61.

The first electrode member 70 includes a first detection electrode plate71, a first reference electrode plate 72, first shield electrode plates73 and 74 which are respectively erected from side edges of the firstreference electrode plate 72, and first inter-electrode charging plates75 and 76.

The second electrode member 80 includes a second detection electrodeplate 81, a second reference electrode plate 82, second shield electrodeplates 83 and 84 which are respectively erected from side edges of thesecond reference electrode plate 82, and second inter-electrode chargingplates 85 and 86.

In the first electrode member 70, the first detection electrode plate 71and the first inter-electrode charging plates 75 and 76 are insulatedfrom each other by an insulating member (not shown) filled in the case61. Further, these plates 71, 75 and 76 are also insulated from thefirst reference electrode plate 72 and the first shield electrode plates73 and 74 (which are formed integrally with the first referenceelectrode plate 72 so as to achieve electrical connection) by theinsulating member. Similarly, in the second electrode member 80, thesecond detection electrode plate 81 and the second inter-electrodecharging plates 85 and 86 are insulated from each other by theinsulating member in the case 61. Further, these plates 81, 85 and 86are also insulated from the second reference electrode plate 82 and thesecond shield electrode plates 83 and 84 (which are formed integrallywith the second reference electrode plate 82 so as to achieve electricalconnection) by the insulating member.

Each of the first shield electrode plates 73 and 74 acts as directivitycontrol means for controlling (limiting) the directions of the electriclines of force of the first detection electrode plate 71. Similarly,each of the second shield electrode plates 83 and 84 acts as directivitycontrol means for controlling (limiting) the directions of the electriclines of force of the second detection electrode plate 81. According tothe arrangements described above, the electric lines of force thatextend sideways of the detection electrode plates 71 and 81 are brokenby each shield electrode plate, thus making it possible to limit only tothe electric lines of force that extend in the upward direction of thedetection electrode plates 71 and 81. Consequently, it is possible torestrict the detection region of the detection electrode plates 71 and81 in the direction of the chargeable member 54.

Accordingly, by providing the first and second shield electrode plates73 and 84, it is possible to prevent an erroneous detection of apedestrian passing by the fence. In addition, by providing the first andsecond shield plates 74 and 83, it is possible to exclude mutualinfluence between the first detection electrode plate 71 and the seconddetection electrode plate 81.

On the surface of the chargeable member 54, a water repellent layer (notshown in the drawings) formed of a water repellent material thatincludes a synthetic resin as a main component is provided to preventinfiltration of moisture into the chargeable member 54. Further, the topsurface of the chargeable member 54 is formed into a roof shape in orderto prevent collection of rain water in the upper part of the chargeablemember 54.

Next, the overall structure of the fence sensor of this embodiment willbe described. The first detection electrode plate 71 and the seconddetection electrode plate 81 are connected to a detection circuit 90shown in FIG. 8, and the first reference electrode plate 72 and thesecond reference electrode plate 82 are grounded. The detection circuit90 is connected to a control circuit (not shown in the drawings). Forthe control circuit in this embodiment, the control circuit of thesecond embodiment is utilized.

First, the detection circuit 90 will be described with reference to FIG.8. The detection circuit 90 includes a pulse signal generator 91, avariable resistor 92, a first variable delay circuit 93, a secondvariable delay circuit 94, and a phase discrimination circuit 95.

A pulse signal output from the circuit 91 is branched via the variableresistor 92 to the first variable delay circuit 93 and the secondvariable delay circuit 94. The first detection electrode plate 71 isconnected to the first variable delay circuit 93, and the seconddetection electrode plate 81 is connected to the second variable delaycircuit 94. The first variable delay circuit 93 delays the input pulsesignal in response to the electrostatic capacitance between the firstdetection electrode plate 71 connected thereto and the first referenceelectrode plate 72, and then sends the result to the phasediscrimination circuit 95 which serves as a comparison means. Similarly,the second variable delay circuit 94 delays the input pulse signal inresponse to the electrostatic capacitance between the second detectionelectrode plate 81 connected thereto and the second reference electrodeplate 82, and then sends the result to the phase discrimination circuit95.

The phase discrimination circuit 95 compares the phases of the pulsesignals sent from the first and second variable delay circuit 93 and 94,and sends out a detection signal to the control circuit when it detectsa phase shift which exceeds a predetermined threshold.

Next, operations of the fence sensor in this embodiment will bedescribed. When an intruder tries to put his hands on the upper part ofthe chargeable member 54 from the side of the sidewall 53 situated onthe outside of the lot, the chargeable member 54 is electrified, and adielectric polarization is generated. The charges generated by thedielectric polarization are distributed more heavily in the portionsituated on the outside of the lot of the chargeable member 54.

The charge distribution of the chargeable member 54 affects also thecharge distribution on the rear surface of the chargeable member 54,with a result that the quantity of polarized charge in the vicinity ofthe first detection electrode plate 71 is larger than the quantity ofpolarized charge in the vicinity of the second detection electrode plate81. Because of this, the strength of the electric field formed on therear surface of the chargeable member 54 varies locally depending uponthe charge quantity. As a result of the electrostatic inductiongenerated by the electric field whose strength varies locally, thecharge quantity on the first detection electrode plate 71 becomes largerthan the charge quantity on the second detection electrode plate 81.

Accordingly, the electrostatic capacitance between the first detectionelectrode plate 71 and the first reference electrode plate 72 becomeslarger than the electrostatic capacitance between the second detectionelectrode plate 81 and the second reference electrode plate 82. As aresult, the phase discrimination circuit 95 discriminates that the pulsesignal from the first variable delay circuit 91 is delayed with respectto the pulse signal from the second variable delay circuit 94, and thensends out a detection signal to the control circuit.

On the contrary, if a resident tries to put on his hands on thechargeable member 54 from the side of the sidewall 52 situated on theinside of the lot, the electrostatic capacitance between the seconddetection electrode plate 81 and the second reference electrode plate 82becomes larger than the electrostatic capacitance between the firstdetection electrode plate 71 and the first reference electrode plate 72.As a result, the phase discrimination circuit 95 discriminates that thepulse signal from the second variable delay circuit 94 is delayed withrespect to the pulse signal from the first variable delay circuit 93. Insuch a case, the phase discrimination circuit 95 will not send out adetection signal to the control circuit. By providing the first andsecond detection electrode plates 71 and 81 in the way described above,it is possible for the fence sensor of the embodiment to detect only anintruder from the outside of the lot.

Next, a fourth embodiment of the fence sensor according to the presentinvention will be described with reference to FIG. 9. The fence sensorin this embodiment is used in a crime prevention system that uses ahandrail 101 in the terrace of an apartment house as a detectionelectrode.

A fence includes a handrail 101 and support pillars 103 for supportingthe handrail 101. The support pillars 103 are provided on a concretebody 104 via an insulating member 102 made of a synthetic resin. Thehandrail 101 is connected to a detection circuit 108 via a lead wire105. In this regard, it is preferable that a shielded wire is used forthe lead wire in order for this part to be immune to the effect ofvariations in the external electric field.

A neon tube 106 is connected between the lead wire 105 and the ground,and a capacitor 107 is connected in series. The neon tube 106 serves asan electrostatic spark preventing means for preventing an excessivecurrent caused by the occurrence of electrostatic sparks from flowinginto the capacitor 107.

In this embodiment, the threshold of the detection circuit 108 is setsuch that a detection signal is output when the electrostaticcapacitance between the detection electrodes in the static state exceeds100 pF. For an electrostatic capacitance of 10,000 pF of the handrail,if the electrostatic capacitance of the capacitor is set at 100 pF, theelectrostatic capacitance as seen from the detection circuit 108 in anapproximately static state can be reduced to slightly below 100 pF.

According to the fence sensor in this embodiment, the capacitor 107 isdisposed at a spot removed from the handrail 101 where changes in thetemperature and humidity are moderate, and the capacitor 107 isconnected to handrail 101 via the lead wire 105. By arranging thecapacitor 107 in such a way, it is possible to minimize the temperaturerise of the capacitor 107, even in the case where the handrail 101 isinstalled in a place where it is exposed to the direct sunlight andwhere the temperature and humidity are liable to change sharply. Namely,it is possible to prevent variations in the electrostatic capacitance ofthe capacitor 107 that are caused by the effect of the temperature orthe like on the dielectric and other parts inside the capacitor 107,thus making it possible to prevent malfunctions of the detection circuit108.

Now, in this embodiment, the electrostatic capacitance of the handrail101 in the static state varies depending upon various conditions such asthe length of the handrail 101, the thickness of the insulator 102, andthe like. However, the fence sensor of this invention is designed so asto be able to adjust the electrostatic capacitance of the capacitor 107,and therefore it is possible to adjust the electrostatic capacitance ofthe capacitor 107 to a proper value where the detection circuit 108 canproperly operate. This means there is no need for adjusting thethreshold of the detection circuit 108. For the reasons described above,according to the present invention, it is possible to easily constructthe fence sensor. Further, it is also possible to quickly install thefence sensor on the construction.

Next, a fifth embodiment of the fence sensor according to the presentinvention will be described with reference to FIG. 10. In thisconnection, the embodiment given below relates to the insulating member102 of the fourth embodiment of the fence sensor.

In the periphery of the insulating member 102, a draining wall 110 whichacts as water film separating means is overhung downward at the upperend part of the support pillar 103. Inner wall surface 111 of thedraining wall 110 is positioned with a distance L1 (more than 6 mm)apart from the side face 112 of the support pillar 103, and a trench 113is formed between the inner wall surface 111 and the side face 112 ofthe support pillar 103.

When a film of rain water or the like is formed on respective surfacesof the handrail 101 and the support pillar 103 of the fence, theelectrostatic capacitance of the handrail 101 increases suddenly justbefore the water film on the surface of the handrail 101 and the waterfilm on the surface of the support pillar 103 come into contact. This isbecause the distance between both water films becomes extremely small,and a state similar to the state in which the handrail 101 is broughtclose to the ground is realized. In this case, the detection circuit 108outputs a detection signal because of a sudden increase in theelectrostatic capacitance of the handrail 101, and then the systemcommits a malfunction. However, by providing the draining wall 110 inthe periphery of the insulating member 102, as in this embodiment, it ispossible to prevent the contact of both water films formed on respectivesurfaces of the handrail 101 and the support pillar 103. This is becauseit is possible, by setting the distance L1 of the trench 113 to be morethan 6 mm, to prevent both water films from contacting across the trench113 under the action of the surface tension. Accordingly, themalfunction of the detection circuit 108 can be prevented.

Next, a sixth embodiment of the fence sensor according to the presentinvention will be described with reference to FIG. 11. This embodimentrelates to a fence sensor having means like the water film separatingmeans in the fifth embodiment.

In the periphery of the insulating member 102, a draining wall whichserves as water film separating means is overhung downward from theupper end part of the support pillar 103. The upper inner wall surface122 of the draining wall 120 is positioned with a distance L2 apart fromthe side face 124 of the support pillar 103, and an auxiliary trench 121is formed between the upper inner wall surface 122 and the side face 124of the support pillar 103. In this connection, the distance L2 should beequal to 1 mm, or more than 1 mm and less than 6 mm. The condition forL2 to be more than or equal to 1 mm comes from the requirement that itshould be larger than the thickness of the water film, and the conditionfor less than 6 mm comes from the requirement that it is necessary tomake it smaller than the distance L1 in the above. The lower inner wallsurface 123 of the draining wall 120 is positioned with a distance L1(more than or equal to 6 mm) apart from the side face 124 of the supportpillar 103, and a main trench 125 is formed between the lower inner wallsurface 123 and the side face 124 of the support pillar 103.

When wind and rain blow sideways to the support pillar 103, a water filmW2 creeps upward on the surface of the support pillar 103. However, whenthe upper end of the water film W2 reaches the auxiliary trench 121, thewater film W2 that tries to go up further is pushed back in theauxiliary trench 121 by the self-weight of the water film W2, and thefurther rise of the water film W2 is prevented. Because of this, theseparation of water films W1 and W2 is maintained. Accordingly,according to the fence sensor in this embodiment, it is possible toprevent malfunctions of the detection circuit 108 even under wind andrain blowing sideways.

Next, a seventh embodiment of the fence sensor according to the presentinvention will be described with reference to FIGS. 12-15. Thisembodiment relates to a fence sensor which utilizes an existingguardrail, and which is used for detecting a vehicle, a pedestrian orthe like that makes approach to the guardrail.

A fence sensor 130 includes a guardrail part 131 made from iron plate,and a plurality of iron support pillars 132 supporting the extendingguardrail part. On the periphery of the support pillar 132, a supportmember 134 equipped with support plates 133 on the left and right isfixed with bolts (not shown) or the like. The bolt is connectedelectrically to the support member 134, while the bolt is insulated fromthe support pillar 132 with an insulating member (not shown) such as aspacer made of an insulator. Further, the bolt is connected to adetection circuit (not shown) via a lead wire. Since the guardrail part131 is fixed to the support plate 133, and is connected electrically tothe support plate 133 and the support member 134, the entirety of theguardrail part forms a detection electrode.

Since the lower part of the support pillar 132 is buried in the groundor the like, the support pillar 132 is grounded. Because of this, thesupport pillar 132 and the support member 134 are insulated with aprescribed distance apart with a rubber insulating member 135 having asemicylindrical shape. However, just before the water films on thesurface of the support member 134 and the support pillar 132 make acontact, the detection circuit triggers a malfunction. For this reason,in this embodiment, four water film separation means are provided in theinsulating member 135. A first water film separation means is an uppersurface trench 142 of an upper part water film separation member 140.The upper surface trench 142 is formed between a draining wall 141 andthe surface of the support pillar 132. Both ends of the upper surfacetrench 142 are left open. Further, the bottom surface of the uppersurface trench is sloped from the central part toward both ends, so thatwater that falls from the top portion of the support pillar 132 isdrained from both ends of the upper surface trench 142. This structureprevents infiltration of water into the surface of the insulating member135. A second water film separation means is a lower surface trench 143of the upper water film separation member 140. The width of the trenchis more than 6 mm. According to the second water film separation means,it is possible to achieve the same results those achieved by thedraining wall 110 that is a water film separation means in the fifthembodiment. Namely, the lower surface trench 143 maintains separation ofa water film on the surface of the support pillar 132 and a water filmon the surface of the insulating member 135. A third water filmseparation means is a side part water film separation member 136 thatprevents the infiltration of water film into the surface of theinsulating member 135 from the sideway of the insulating member 135. Afourth water film separation means is a lower trench 137 of a lowerwater film separation means 138. The width of the trench is more than 6mm. According to the fourth water film separation means, it is possibleto achieve the same results those achieved by the lower trench 143 ofthe water film separation trench 140. Namely, the lower trench 137maintains the separation of a water film on the surface of theinsulating member 135 and a water film on the surface of the supportpillar 132.

Next, an eighth embodiment of the fence sensor according to the presentinvention will be described with reference to FIG. 16. This embodimentdirected to a fence sensor that utilizes conductive chains 151 and 152which are strung between a plurality of support pillars 132 as detectionelectrodes, and that is used for detecting an object that makes approachto the chains.

In this connection, it is to be noted that in this embodiment thestructure of water film separation means is generally the same as in thesixth embodiment, and elements having the same functions will beindicated by the same numerals.

In this embodiment, chain fixing members 153 on the periphery of thesupport pillar 132 are insulated from the support pillar 132 with aspacer made of an insulator (not shown), and are connected to adetection circuit (not shown) via a lead wire. Accordingly, the entiretyof the chain fixing members 153 that are formed of a conductive metaland the chains 151 and 152 forms a detection electrode.

Next, the fence sensor according to a ninth embodiment of the inventionwill be described with reference to FIGS. 17 and 18. The fence sensor inthis embodiment utilizes a movable iron fence 160 that is used on aconstruction site or the like as a chargeable member. This fence sensoris designed to detect human being or the like that makes approach to thefence 160 by a detection electrode installed in a detection leg part170. In the upper part of the detection leg part 170, there are providedan engaging hole 176 that engages with the end part of a support pillar161, and water film separation means 172 having a trench 173 formed inits periphery. The water film separation means 172 separates water filmson the surface of the fence 160 acting as a chargeable member and itssupport pillar 161, and a water film on the surface 171 of the lowerpart of the detection leg part 170. Inside the detection leg part 170,there are arranged a detection electrode 174 and a ground electrode 175that face with each other. In this embodiment, the threshold of adetection circuit (not shown) is adjusted such that an end of thesupport pillar 161 in the engaging hole 176 is positioned within thedetection region of the detection electrode 174. In this connection,each of the leg parts of the other three support pillars 161 (other thanthe one on which the detection leg part 170 is mounted) is provided witha height adjustment member 162 for equalizing their height with that ofthe detection leg part 170.

INDUSTRIAL UTILIZATION

As described in the above, the fence sensor according to the presentinvention can be utilized mainly as a sensor for crime prevention fordetecting an intruder or the like.

What is claimed is:
 1. A fence sensor, comprising: a detectionelectrode; a reference electrode insulated from the detection electrode;a chargeable member insulated from both of the detection electrode andthe reference electrode, the chargeable member being arranged such thatat least a part of the chargeable member is situated within a detectionregion of the detection electrode, and the chargeable member beingformed from a conductor; and a detection circuit for detecting a changein the electrostatic capacitance between the detection electrode and thereference electrode that is generated by the presence of an object to bedetected within the detection region.
 2. The fence sensor as claimed inclaim 1, further comprising water film separation means for separating awater film on the surface of the chargeable member from a water filmgrounded to the earth.
 3. A fence sensor, comprising: a detectionelectrode; a reference electrode insulated from the detection electrode;a chargeable member arranged such that at least a part of the chargeablemember is situated within a detection region of the detection electrode,the chargeable member being formed from an insulator; and a detectioncircuit for detecting a change in the electrostatic capacitance betweenthe detection electrode and the reference electrode that is generated bythe presence of an object to be detected within the detection region. 4.The fence sensor as claimed in claim 1 or 3, wherein the detectionelectrode and the reference electrode are partially or completelyconcealed by the chargeable member.
 5. The fence sensor as claimed inclaim 1 or 3, wherein the reference electrode is connected electricallyto the ground or a building.
 6. The fence sensor as claimed in claim 1or 3, wherein the chargeable member is provided with water repellentmeans.
 7. The fence sensor as claimed in claim 1 or 3, furthercomprising directivity control means for limiting the direction of theelectric lines of force of the detection electrode.
 8. The fence sensoras claimed in claim 7, wherein the directivity control means is ashielded electrode connected to the reference electrode.
 9. The fencesensor as claimed in claim 1 or 3, further comprising at least oneinter-electrode chargeable member which is disposed between thedetection electrode and the reference electrode, and which is insulatedfrom both of the detection electrode and the reference electrode. 10.The fence sensor as claimed in claim 1 or 3, wherein the detectionelectrode includes first and second detection electrodes which areinsulated from each other, and wherein the detection circuit includescomparison means for comparing electrostatic capacitance between thefirst detection electrode and the reference electrode with electrostaticcapacitance between the second detection electrode and the referenceelectrode.
 11. The fence sensor as claimed in claim 1 or 3, wherein thedetection electrode and the reference electrode are constructed from aplurality of sets of detection electrode and reference electrode, inwhich the plurality of the detection electrodes are electricallyconnected to each other, the plurality of the reference electrodes areelectrically connected to each other, and the plurality of detectionelectrodes and the plurality of the reference electrodes are connectedto the detection circuit.
 12. A fence sensor, comprising: a detectionelectrode; a reference electrode insulated from the detection electrode;a detection circuit for detecting a change in electrostatic capacitancebetween the detection electrode and the reference electrode that isgenerated by the presence of an object to be detected within thedetection region; and a capacitor connected in series between thedetection circuit and the detection electrode, the capacitor beingdisposed separated from the detection electrode.
 13. The fence sensor asclaimed in claim 12, further comprising electrostatic spark preventingmeans which is disposed between the detection circuit and the detectionelectrode.
 14. A fence sensor, comprising: a detection electrode; areference electrode insulated from the detection electrode; a detectioncircuit for detecting a change in electrostatic capacitance between thedetection electrode and the reference electrode that is generated by thepresence of an object to be detected within the detection region; andwater film separation means for separating a water film on the surfaceof the detection electrode from a water film grounded to the earth. 15.The fence sensor as claimed in claim 2 or 14, wherein the water filmseparation means has a trench with a width of 6 mm or more, in which thetrench is opened downward.
 16. The fence sensor as claimed in claim 2 or14, wherein the water film separation means has a main trench with awidth of 6 mm or more, and an auxiliary trench with a width of less than6 mm, in which the main trench is opened downward, and the auxiliarytrench is opened downward and provided in the main trench.
 17. A fencesensor, comprising: a detection electrode; a reference electrodeinsulated from the detection electrode; and a chargeable memberinsulated from both of the detection electrode and the referenceelectrode, the chargeable member being formed from a conductor or aninsulator, and the chargeable member being arranged such that at least apart of the chargeable member is situated within a detection region ofthe detection electrode.
 18. A sensor for an ascending and descendingmember, comprising: a detection electrode; a reference electrodeinsulated from the detection electrode; an ascending and descendingmember insulated from both of the detection electrode and the referenceelectrode, the ascending and descending member being formed from aconductor or an insulator, and the ascending and descending member beingarranged such that at least a part of the ascending and descendingmember is situated within a detection region of the detection electrode;and a detection circuit for detecting a change in electrostaticcapacitance between the detection electrode and the reference electrodethat is generated by the presence of an object to be detected within thedetection region.
 19. A sensor for an ascending and descending member,comprising: a detection electrode; a reference electrode insulated fromthe detection electrode; and a chargeable member insulated from both ofthe detection electrode and the reference electrode, the chargeablemember being formed from a conductor or an insulator, and the chargeablemember being arranged such that at least a part of the chargeable memberis situated within a detection region of the detection electrode.